A drive roller unit for driving an article such as a freight container on a conveyor track, as described in U.S. Pat. No. 4,792,037 comprises a drive roller which can be driven in rotation by an electric motor and which is movable by means of a guide means relative to the base of the unit between a lower rest position and an upper drive position in which it is in engagement with the underside of the article to be driven. The drive roller is substantially in the form of a hollow circular cylinder, the interior of which accommodates the electric motor, with the rotor of the motor being mounted rotatably within the stator of the motor. The stator of the electric motor, which includes the field winding, is rigidly and non-rotatably connected to a stator housing which serves as a guide body. The guide body has a circular-cylindrical outside peripheral surface which is eccentric with respect to the axis of rotation of a drive gear, which axis is stationary with respect to the base of the unit. The drive roller is mounted rotatably on the outside surface of the guide body. With that design configuration, when the motor is switched on, the rotor which is disposed in the interior of the stator initially remains in a stationary condition because of its inherent inertia and by virtue of a step-down transmission which applies a braking effect thereto, and a special brake. As a result the motor torque acting between the rotor and the stator rotates the stator arrangement and therewith the guide body formed by its housing, around the axis of rotation of the drive gear which meshes with an internal tooth arrangement in the drive roller. As a result, because the guide body is eccentric with respect to the axis of rotation of the drive gear, the drive roller which is mounted thereon is moved out of its lower rest position into its upper operative position in which it comes to bear against the bottom of the article to be conveyed, which is disposed above it.
A further drive roller unit as disclosed in U.S. Pat. No. 3,698,539 has a base or main frame which is to be secured for example to the floor of an aircraft freight compartment, and a mounting frame structure which is mounted at one end pivotably to the base or main frame. In the vicinity of the pivot axis the mounting frame structure carries a motor while at a spacing relative to the pivot axis it has a drive roller which is mounted in such a way as to be rotatable about an axis which is stationary with respect to the mounting frame structure, as well as a cam control arrangement which is connected to the free end of the mounting frame structure and by means of which the mounting frame structure is pivotable relative to the main frame from the rest position into the operative position. The output shaft of the motor is connected by way of a transmission train to the input of a planetary transmission, of which one output is coupled by way of a transmission arrangement to the drive shaft of the drive roller while the other output of the planetary transmission drives the cam control unit.
In the rest condition the mounting frame structure lies on the main frame so that the drive roller is disposed in a lowered condition at a spacing from the bottom of a freight container positioned above same on a roller conveyor track. In order for the freight container to be set in motion on the roller track, the motor is firstly switched on, to drive the input of the planetary transmission. At that time the drive roller is prevented from rotating by means of a slipping clutch with a predetermined braking torque so that the drive moment of the motor inevitably acts by way of the other output of the planetary transmission on the cam control unit which, as a result, by virtue of its cams bearing against the main frame, pivots the mounting frame structure upwardly into the operative position until the drive roller comes into engagement with the underside of the bottom of the freight container. The weight of the freight container causes the movement of the cam control unit to be blocked thereby, with the drive roller being pressed against the bottom of the container. As a result the output of the planetary transmission which drives the cam control unit is also blocked so that the other output of the planetary transmission, which is coupled to the drive roller, transmits to the drive roller a braking torque which overcomes the braking torque of the slipping clutch so that the drive roller, while being pressed against the bottom of the container, begins to rotate and the container is moved forwardly in a direction which depends on the direction of rotation of the drive motor.
A problem which arises with that arrangement in a practical situation is that the entire transmission arrangement from the motor to the drive roller on the one hand and to the cam control assembly on the other hand is of a very expensive design configuration and comprises a large number of components. In addition a cam control assembly is required so that the overall construction involves a large number of mechanically moved components. As a result, not only does the arrangement involve a high level of manufacturing costs but it is also of large size and high weight which is extremely undesirable in regard to aircraft construction. In addition the large number of components which move relative to each other gives rise to susceptibility to wear and short periods of time between maintenance operations are required in order to guarantee the high level of reliability required in operating aircraft.
In particular in the above-discussed drive roller unit the individual components are disposed more or less without protection in a juxtaposed arrangement so that in operation it is not only necessary to reckon on the assembly suffering from fouling and operational faults which are caused thereby, but it is also susceptible to damage caused by objects penetrating into the mechanism or due to the direct action of a force thereon.
A drive roller unit which is similar in terms of design and mode of operation is also to be found in U.S. Pat. No. 4,697,694. However besides being of large size, that arrangement may also suffer in operation from problems due to foreign bodies penetrating into the mechanism or due to the effect of a force acting thereon.